1. Field of the Invention
This invention relates to stable high conductivity functionally gradient compositionally layered solid state electrolytes and membranes providing improved oxygen-ion conductivity for electrolytes and improved mixed oxygen-ion and electronic conductivity for membranes. The electrolytes of this invention are useful in solid oxide fuel cells and as sensors. The membranes of this invention are useful in gas separation and in membrane reactors. Multi-layered solid electrolytes and membranes of this invention have a mixed conducting anodic portion with high oxygen ionic conduction and electronic conduction on one side, preferably of the n-type, which may be exposed to a reducing atmosphere, and interface on the opposite side with a high oxygen ionic conducting and low electronic conducting portion for the electrolyte and high p-type ionic conducting for the membrane, and exposed to an oxidizing atmosphere on the opposite side.
2. Description of Related Art
Oxygen ion conductors based upon Bi.sub.2 O.sub.3 have been known for solid electrolytes and oxygen separation due to their high oxygen ion conductivity and their ability to operate at lower temperatures. There have been various attempts to improve their phase stability and their thermodynamic stability against reduction. U.S. Pat. Nos. 5,006,494 and 5,183,801 teach the phase stability of Bi.sub.2 O.sub.3 in the cubic form stabilized by 10 to 40 mole percent of a rare earth oxide, such as yttria, is enhanced by inclusion of up to 10 mole percent of an oxide of a cation having a valence of 4 or greater, such as zirconia, hafnia, thoria, stannic oxide, tantalum oxide, and niobium oxide as a dopant. It has been shown that Bi.sub.2 O.sub.3 stabilized in the .delta.-phase by 20 mol % Er.sub.2 O.sub.3 has ionic conductivity in the order of 1 to 2 orders of magnitude greater than that of yttria stabilized zirconia at comparable temperatures. E. D. Wachsman, N. Jiang, D. M. Mason, and D. A. Stevenson, Proceedings of the First International Symposium on Solid Oxide Fuel Cells, Electrochem. Soc., 89-11, 15 (1989). Bi.sub.2 O.sub.3 has been reported to exhibit purely ionic conduction down to low partial pressures of oxygen of 10.sup.-20 atm. E. D. Wachsman, G. R. Ball, N. Jiang, and D. A. Stevenson, Solid State Ionics, 52, 213 (1992); C. Wang, X. Xu, and B, Li, Solid State Ionics, 13, 135, (1984); and P. Duran, J. R. Jurado, C. Moure, N. Valverde, and B. C. H. Steele, Mat. Chem. Phys., 18, 287 (1987). Ionic conduction at low oxygen pressure has been observed only in the absence of a reducing agent. The reducing environments of fuels, such as methane and hydrogen, decompose Bi.sub.2 O.sub.3 to metallic bismuth. Wachsman, Ball, Jiang, and Stevenson, supra.
Problems of the reducibility of Bi.sub.2 O.sub.3 electrolytes has been recognized and various attempts have been made to solve the problem. U.S. Pat. No. 5,213,911 teaches a solid oxide low temperature electrolyte with very low electron conduction and stable in H.sub.2 formed by combination of a compound having weak intramolecular metal-oxygen interactions with one having stronger metal-oxygen interactions.
A graded zirconia-bismuth oxide electrolyte is taught by U.S. Pat. No. 5,171,645 having a Bi.sub.2 O.sub.3 doped for increased oxygen ion transport rich layer on one side and a ZrO.sub.2 doped for increased oxygen ion transport rich layer on the opposite side with gradations of the components between, enabling low temperature operation and better thermal expansion stability in a solid oxide fuel cell. In another embodiment, both surfaces are doped ZrO.sub.2 rich layers graded to a relatively thick doped Bi.sub.2 O.sub.3 rich central layer.
Aliovalent doped ceria in the fluorite structure is also known to exhibit ionic conductivity significantly greater than yttria stabilized zirconia at comparable temperatures. R. N. Blumenthal, F. S. Brugner, and J. E. Garnier, J. Electrochem. Soc., 120, 1230 (1973). However, ceria electrolytes have a high electronic conduction in a reducing environment. U.S. Pat. No. 5,001,021 teaches limiting electronic conduction in ceria electrolytes by double metal doping.
A mixed oxygen ion and electronic conductor may favor n-type electronic conduction in a reducing environment while in an oxidizing environment it may favor p-type conduction. Thus, in theory a mixed conductor may exhibit pure ionic conduction over a central region between the n-type and p-type regions. I. Riess, J. Electrochem. Soc., 128, 2077-2081 (1981) and I. Riess, Solid State Ionics, 52, 183 (1992). The use of mixed conductors for the entire anode/electrolyte/cathode structure has been investigated, but no significant electrolytic region was detected. P. Han, R. Mukundan, O. K. Davis, and W. L. Worrell, Mixed Conducting Oxides in Solid Fuel Cells, EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, Mar. 22-23, 1994, Atlanta, Ga. This work was an extension of single component solid oxide bodies with n-type conductance on one side and p-type conductance on the opposite side of an oxygen ion conducting central portion taught by U.S. Pat. No. 5,298,235 wherein appropriate conduction is induced by chemical modification, doping, or by chemically tailoring electronic conducting perovskites to produce an oxygen ion conducting electrolyte central portion.
Use of ionic conductors for electrocatalytic chemical reactions is known. U.S. Pat. Nos. 4,793,904; 4,933,054; and 4,802,958 teach a solid electrolyte of Bi.sub.2 O.sub.3 stabilized by a lanthanide or calcium oxide having a conductive coating on one side capable of reducing oxygen to oxygen ions and a conductive coating on the opposite side capable of desired oxidative catalysis. Use of mixed ionic-electronic conductors for electrocatalytic chemical reactions is taught by U.S. Pat. No. 5,273,628 which teaches thin membranes of homogeneous solid solutions and non-homogeneous mixtures of Bi.sub.2 O.sub.3 stabilized with a stabilizer, such as Y, and containing a variable valence j metal, such as Ti. U.S. Pat. No. 5,306,411 teaches multi-phase mixtures of an electronically conductive material and an oxygen ion conductive material based upon ABO.sub.3 perovskites, preferably containing small amounts or no bismuth.
Problems of physical stability of solid oxygen conducting electrolytes have been recognized. U.S. Pat. No. 5,069,987 teaches a solid oxide fuel cell having an electrolyte with repeating array of ductile ordered, continuous metallic fibers imbedded in the ceramic matrix, such as expanded Ni foil.
Mixed oxygen ion and electronic conductors have been used in electrodes as taught by U.S. Pat. No. 5,364,506 which teaches a stabilized ZrO.sub.2 electrolyte and mixed conducting perovskite anode; U.S. Pat. No. 4,931,214 teaches an electrode of ZrO.sub.2 with an oxide stabilizing agent, such as Y.sub.2 O.sub.3, and a metal oxide, such as TiO.sub.2 ; U.S. Pat. No. 4,812,329 teaches solid oxygen conducting electrolytes with bonded anodes of particles of electronic conductor partly embedded in a skeleton of ceramic metal oxide with its surface covered with a separate, porous, gas permeable mixed oxygen ionic-electronic conducting coating; and U.S. Pat. No. 3,956,194 teaches positive electrodes of monophased graphite having an alkali cation, a transition metal, and a non-metallic electronegative atom.
U.S. Pat. No. 5,240,480 teaches an oxygen ion conducting membrane having a porous layer with average pore sizes of less than about 10 microns and a contiguous dense non-porous layer, both materials being a mixed ionic-electronic conducting multicomponent metallic oxide, such as perovskites.